Most air purification filters work by trapping contaminants. Contaminants can include particles such as dust, or biological or chemical contaminants in vapor form. Filters contain numerous “free sites”. As contaminants attach to the filter, these free sites become occupied and exhausted. When a large proportion of free sites are exhausted, the filter is no longer effective in removing 100% of the contaminants from the air stream. Therefore the filter must be changed. Determining when a significant number of sites is exhausted and when a filter must be changed is a difficult task.
Historically, most filter change guidelines have been based on time in service. Under this method filters are changed after they have been in service for a certain period of time. This method can result in filters being changed early or late depending on the speed of airflow and the level of contamination in the air stream.
Other prior art systems that determine at which time a filter should be changed use a sample canister. One such example is disclosed in U.S. Pat. No. 4,135,896, incorporated herein by reference in its entirety. Sample canisters can attach to outlet ends of a housing of a filter bed. Rather than flowing through filter beds, a small portion of contaminated gas passes through the sample canister. The sample canister includes a filter bed having a width that is equal to or slightly less than the width of the gas purifier filter bed. Accordingly an exhaustion of free sites in the sample canister should coincide with an exhaustion of free sites in the gas purifier filter bed, everything else being equal.
One problem with the above-mentioned indicating system is that the contaminated air must enter the filter housing before entering the sample canister for the indicating system to work. Furthermore, the volume of contaminated air passing through the canister may not be proportional to the volume of air passing through the filter media. This could result in false comparisons between the number of free sites remaining in the filter bed of the filter being monitored and the number of free sites remaining in the sample canister's filter bed.
Accordingly, it is desirable to provide an air purification assembly including a system that can perform the following functions: monitor the filter bed, indicate how much filter life remains and determine when the filter should be changed. It is also desirable to provide a residual life indicating system that can be used with different types of air purification systems.